The present invention generally relates to photolithography systems used in processes for manufacturing semiconductor devices, and deals more particularly with a method and apparatus for precisely controlling the dose of radiation to which a semiconductor wafer is subjected during exposure imaging.
Modem fabrication processes for producing semiconductor devices, such as integrated circuits, have long employed photolithography for transferring circuit patterns onto a semiconductor substrate, such as a wafer. In general, photolithography involves the performance of a sequence of process steps, including coating a semiconductor wafer with a resist layer, exposing the coated wafer to a patterned light source, developing the resist layer, processing the semiconductor wafer through the developed resist layer, and removing the resist layer. An optical photolithography stepper apparatus, sometimes referred to as a xe2x80x9cstep and scanxe2x80x9d or xe2x80x9cstepperxe2x80x9d, is typically used to expose the resist layer. An image of each layer of an IC die is formed on a small rectangular piece of glass referred to as a reticle or mask. The mask or reticle is placed on the stepper and a reduced image thereof is projected onto a portion of the resist layer covering the semiconductor wafer. Specifically, the reticle patterns are transferred to the wafer by scanning the patterns through a narrow illumination slit.
When numerous IC""s are to be fabricated from a single wafer, a mask used in the fabrication of any one IC is also used in the fabrication of the other IC""s from the wafer. This is accomplished by using the stepper to index or xe2x80x9cstepxe2x80x9d lie wafer under an optical system which includes the mask or reticle. At each step, the photoresist is exposed to the optical system, typically with ultraviolet light, to form an aerial image of the mask on the layer of photoresist. The wafer is then removed from the stepper and the image is developed. At that point, the wafer is etched to remove portions of the underlying film, following which the wafer is ready for the next stage of processing, which might include for example, ion implantation, deposition or other types of etching processes. At a later stage in the fabrication process, the wafer is returned to the stepper for exposure of the wafer to a different mask.
The reticle or mask is composed of a glass substrate, such as quartz, on which there is formed a circuit pattern composed of materials such as chromium which prevents ultraviolet light rays from transmitting therethrough. The reticle is set in the stepper in order to expose a semiconductor wafer to light, and the circuit pattern formed on the reticle is imaged by the stepper onto the semiconductor substrate.
Semiconductor manufacturing processes are aimed at achieving up to 0.25 micron resolution in a high production environment. This goal is being driven by the need to develop competitive device performance and lower manufacturing costs per device. In order to increase the field size and improve critical dimensional control below 0.25 micron resolution, improvements is step and scan technology will play a critical role. Improvements in the area of highly controllable, precise light sources, such as excimer lasers with appropriate dose control are important for solving illumination control problems and achieving exceptionally short exposure times.
In order to more precisely control the dose of light radiation projected on the wafer, the illumination system and scanner slit have, in the past, been provided with an adjustment that allows focusing of the image applied to the wafer. This adjustment system relies on movement and adjustment of mechanical elements, and particularly the displacement of the mechanical slit relative to the illumination source. As a result of the dependency on this mechanical adjustment, repeatable results are not always obtained from batch to batch since adjustment settings may change for a number of reasons. Moreover, the need to perform periodic preventive maintenance on equipment introduces the further possibility that adjustment settings may be inadvertently altered, thus making repeatable, precise dosage control possible.
Accordingly, there is a clear need in the art for a system for controlling radiation dosage and intensity profile in a manner that does not rely on mechanical adjustments, and thus provides stable, repeatable dosage control. The present invention is directed towards satisfying this need in the art.
According to one aspect of the invention, apparatus is provided for controlling the dose of radiation exposure applied to a semiconductor wafer during a photolithography process. The apparatus broadly comprises a source of radiation such as ultraviolet light; a set of reticle masking blades for masking a pattern of radiation on to the wafer, means for delivering radiation from the radiation source through the masking blades, and means juxtaposed between the masking blades and the delivery means for shaping the intensity profile of the radiation passing between the blades and reticle, which is applied to the wafer. The shaping means preferably includes an angularly shaped member formed of a material that is either semitransparent to or partially reflective of the radiation. The angularly shaped member preferably possesses a generally rectangular cross section and is formed of Mo Bi Si O4. The shaping member extends around a masking opening defined between opposing sets of the blades. A beam of radiation transmitted from the source passes through the ring so as to alter the two dimensional intensity profile of the radiation, following which the shaped beam passes through the masking blades and reticle so as to image onto the wafer.
According to another aspect of the invention, apparatus is provided for exposing a semiconductor wafer to radiation during a photolithography process, which includes a source of radiation, means for forming the radiation source into a beam, a mask for forming a pattern of the radiation on the wafer, means for directing the beam onto the mask, and means for shaping the intensity of the radiation applied over two dimensions across the wafer. The shaping means include a member formed of partially transparent or translucent material and positioned such that the beam passes through the shaping member. The delivery means preferably includes a quartz rod for carrying the beam, and the mask includes at least two opposing blades disposed downstream of the rod in the optical delivery path of the beam, wherein the shaping member is juxtaposed between the rod and the blades. The rod possesses a circular end through which the beam exits the rod, and the shaping member is angularly shaped and is mounted at the rod end concentric with the rod.
According to another aspect of the invention, a method is provided for controlling the dosage of radiation applied to a semiconductor wafer during a photolithography process, comprising the steps of: generating a beam of radiation; supplying a mask for masking the radiation applied to the wafer; and attenuating portions of the beam by passing the beam through a radiation attenuating member before the beam passes through the mask. The beam is generated by passing radiation through a quartz rod and attenuation is achieved by placing the attenuating member between the rod and the mask.
A still further aspect of the invention comprises a method of controlling the dosage of radiation applied to a semiconductor wafer during a photolithography process used to manufacture the wafer, comprising the steps of: generating a beam of radiation; passing the beam through a ring shaped member formed of a radiation attenuating material, whereby to alter the intensity of the radiation in two dimensions across the beam, and then, passing the attenuated beam through a mask onto the wafer.
Accordingly, it is a primary object of the present invention to provide a method and apparatus for controlling the dose of radiation exposure applied to a semiconductor wafer during a photolithography process employed to image circuit pattern onto the wafer.
Another object of the invention is to provide a radiation dose control method and apparatus as described above which eliminates dependency on mechanical mechanisms and provides highly repeatable results from batch to batch and machine to machine.
A further object of the invention is to provide a method and apparatus of the type mentioned above which allows for precise control of radiation profiles in two dimensions across the surface of the wafer.
A still further object of the invention is to provide a method and apparatus for control of radiation dosage as referred to above which is particularly simple and economical.
These, and further objects and advantages of the present invention will be made clear or will become apparent during the course of the following description of the preferred embodiment of the invention.